The interpolation filter of the present invention is of general utility and is particularly useful for generating additional horizontal lines for display in a "line doubling" or progressively scanned television display system. In one form of progressive scan system the horizontal scan rate is multiplied, i.e., doubled, and each line of video is displayed twice thereby providing a displayed image having reduced visibility of line structure. An example of such a progressively scanned receiver, wherein the added lines of video signal are replicas of the original scan lines, is described in U.S. Pat. No. 4,415,931 entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES which issued Nov. 15, 1983 to R. A. Dischert.
In another form of progressive scan system, the "extra" lines for display are obtained by interpolation of adjacent vertical lines of the incoming video signal. This may be done either before or after "speed-up" (i.e., time compressing) of the video signal in the memory. Examples of progressively scanned display systems in which the additional scan lines are obtained by interpolation from the original scan lines are described by K. H. Powers in U.S. Pat. No. 4,400,719 entitled TELEVISION DISPLAY SYSTEM WITH REDUCED LINE SCAN ARTIFACTS which issued Aug. 23, 1983 and by Yasushi Fujimura et al. in UK Patent application Ser. No. 2,111,343A published June 29, 1983.
It has been recognized (by Powers, for example) that a superior progressive scan image may be obtained in cases where there is no significant scene motion, by using a field memory to delay the incoming video signal by one field. In this way, all 525 lines of an interlaced frame (NTSC assumed) are available for display during each field period thereby avoiding the loss of vertical resolution characteristic of conventional line interpolators. When motion occurs, however, the temporal difference (1/60th. second for NTSC) between the undelayed and field delayed lines causes the edges of moving objects to appear serrated.
The serrated edge effect may be corrected as described by Casey in U.S. Patent application Ser. No. 614,668 filed May 29, 1984 entitled A TELEVISION RECEIVER THAT INCLUDES A FRAME STORE USING NON-INTERLACED SCANNING FORMAT WITH MOTION COMPENSATION, commonly assigned to the assignee of the present invention. In an exemplary embodiment of the Casey apparatus, a motion responsive switch selects a frame comb filtered luminance signal for display during times when little or no motion is present and selects the field-delayed luminance signal (which, additionally, is line-comb filtered) for display otherwise.
Another example of an "adaptive" system that automatically switches between two processing modes is described by Casey et al. in U.S. Patent application Ser. No. 738,974 entitled PROGRESSIVE SCAN PROCESSOR EMPLOYING INTERPOLATION IN THE LUMINANCE CHANNEL filed May 29, 1985. In this system a selector switch selects a field delayed luma image signal for display on the intermediate lines of a display when the vertical detail content of the input signal is greater than a minimum threshold value and there is little or no interframe motion and selects a frame comb filtered and interpolated luminance signal for display otherwise thereby providing enhanced vertical detail and reduced field motion artifacts for displayed images.